The invention relates to intrusion detectors or alarms and, more particularly, to infrared intrusion detectors.
Infrared intrusion detectors are used for the detection of persons or objects moving in a spatial region, by sensing infrared radiation from the persons or objects. Such detectors include one or more infrared sensors, each with two or more pyroelectric sensor elements, which emit an electrical signal with changing incident infrared radiation. The infrared radiation from the spatial region to be monitored passes through an infrared-permeable entrance window into the detector housing and is focused by optical focusing elements onto the infrared sensor elements. Typically, the optical focusing elements are concave mirrors with a plurality of mirror surfaces, or Fresnel lenses at the entrance window. Typically also, the sensor elements are connected differentially in pairs, in order to compensate for the thermal effects of air flows over the sensors or the entrance window.
In order to distinguish infrared radiation from warm bodies from extraneous radiation at other wavelengths, e.g., from visible light from automobile headlights, and thus to guard against false alarms, infrared intrusion detectors are provided with various optical filters. The insensitivity of infrared intrusion detectors to extraneous light is verified by official testing authorities, e.g., by the Association of Property Insurers in the Federal Republic of Germany.
U.S. Pat. No. 3,703,718 discloses an infrared intrusion detector with an optical filter between the focusing mirror and the infrared sensor. The filter transmits radiation in the useful band of 4.5 to 20 micrometers, i.e., the typical body radiation of living organisms. In such a detector, the optical filter may heat up due to absorbed radiation, and may emit secondary radiation in the useful band. This secondary radiation can reach the sensor and trigger a false alarm.
U.S. Pat. No. 5,055,685 discloses an infrared intrusion detector in which secondary radiation from the irradiated optical filter is less likely to trigger a false alarm. An infrared filter is spaced from the infrared sensor element by a sufficient distance, to equalize the intensity of secondary radiation on the two infrared sensor elements from the filter. The resultant difference signal is then approximately zero.
For avoiding false alarms due to extraneous light, Swiss Patent Document 680,687 discloses an entrance window of an infrared intrusion detector which further serves as infrared filter. The window comprises a polyethylene foil in which zinc sulphide particles having a particle size of 0.5 to 50 micrometers are uniformly distributed. The window has high optical transmittance in the wavelength range from 4 to 15 micrometers. Extraneous light, in the visible and near-infrared range, is scattered by the zinc sulphide particles, so that little extraneous light reaches the infrared sensor elements.
Still, these infrared intrusion detectors remain prone to false alarms due to secondary radiation from filters or protective windows, or to heat conducted from the sensor housing to the sensor elements. With increasingly stringent standards to be met, infrared intrusion detectors must be made less likely to produce false alarms due to extraneous light.